摘要:

AbstractA temperature variation study of the nuclear quadrupole resonance (NQR) frequencies for the two resonance lines in 1,2,4,5‐tetrabromobenzene was carried out in the range 77–300 K. The far‐infrared spectrum of 1,2,4,5‐tetrabromobenzene was recorded and the low‐frequency vibrational modes noted. From the nine observed frequencies, the torsional modes contributing to the change in NQR frequencies with temperature are identified by correlating the calculated values of the NQR frequencies at different temperatures with the observ

ISSN:0030-4921    

DOI:10.1002/mrc.1270220102

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractAnalysis of the1H NMR spectra of several monothiocarbonohydrazones, some of them synthesized for the first time, shows that they exist as two structural isomers. Whereas, in general, the derivatives of aromatic aldehydes conform to a linear structure, the aliphatic carbonyl derivatives conform to heterocyclic or linear structures, depending on the size of the substituent groups. This dual behaviour is explained in terms of extended conjugation and steric hindrance.

ISSN:0030-4921    

DOI:10.1002/mrc.1270220103

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of eleven 3‐N,N‐dialkylamino‐5,5‐dimethylcyclohex‐2‐en‐1‐ones have been determined. The chemical shifts of the three sp2carbons and of the two methylene carbons on the cyclohexenone moiety have been subjected to factor analysis. Two factors are necessary and sufficient to account for more than 93% of the total variance. The more important axis (79%) corresponds to a factor closely related to the inductive and steric effects of the alkyl nitrogen substituents. The second parameter is more difficult to interpret and could correspond to the ‘ipsoeffects

ISSN:0030-4921    

DOI:10.1002/mrc.1270220104

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of 28 enethiolizable β‐thioxo esters and 6 enethiolizable β‐thioxo thioloesters have been recorded in order to establish the tautomeric phenotypes of these compounds. All compounds investigated are essentially enethiolic. The carbonyl‐conjugated (Z)‐enethiol form is the exclusive or predominant tautomer of open‐chain β‐thioxo esters and thioloesters, thioacylmalonates and medium‐sized 2‐alkoxycarbonylcycloalkanethiones. The carbonyl‐conjugated (E)‐enethiol form is identifiable for open‐chain α‐unsubstituted β‐thioxo esters and thioloesters, and abundant for open‐chain α‐substituted β‐thioxo esters. Non‐conjugated enethiol forms [i.e. (Z)‐ and (E)‐isomeric β,γ‐unsaturated β‐mercapto esters] are abundant tautomeric constituents of ω‐substituted and higher 2‐alkoxycarbonylcycloalkanethiones. The chemical shifts of the carbon atoms directly involved in the tautomeric change have been rationalized in terms of substituent screening contributions. Deuterium isotope effects on the central carbon atoms of selected deuterio‐enethiolic compounds have been measured in order to depict the ester group rotamerism in CO‐conjugated (Z)‐enethiols. The abundance of the CO‐conjugated (E)‐enethiols, as well as the preferred population of the non‐conjugated (Z)‐enethiol form relative to the non‐conjugated (E)‐enethiol form, is rationalized in terms of the occurrence of a no‐bond interaction between the lone‐pair electrons of the enethioli

ISSN:0030-4921    

DOI:10.1002/mrc.1270220105

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractIt is shown that the complexation of the (R)‐MTPA (α‐methoxy‐α‐trifluoromethyl‐α‐phenylacetic acid) ester ofcis‐4‐tert‐butylcyclohexanol with Eu(fod)3is very similar to that of the correspondingtrans‐ester and ketones with Eu(fod)3. Further evidence is provided that the MTPA moiety exists as two different rotamers. The LIS technique, used as a tool for structure and conformation elucidation, was found by means of a Monte Carlo error analysis not to be dependent on sma

ISSN:0030-4921    

DOI:10.1002/mrc.1270220106

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThesynandantirotamers ofo‐tolyl‐di‐tert‐butylcarbinol, 2a and 2b, respectively, have been studied by1H NMR at 200 MHz and by natural abundance13C NMR at 50 MHz.1H‐{1H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time,T1, of the 2‐Me protons in 2b is unexpectedly higher than that for 2a. The13C relaxation times of the 2‐Me and the quaternary carbon of thetert‐butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies fort‐Bu rotation, measured by1H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2‐Me andt‐Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG‡ values from this work and from the literature with MM2‐calculated ΔH‡ values indicates that this force field systematically underestimates rotation barriers in open‐chain systems by

ISSN:0030-4921    

DOI:10.1002/mrc.1270220107

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractAssignments of the13C NMR signals of the dammarane triterpenes, 3β,25,30‐trihydroxy‐(20R,24R)‐epoxydammaran‐16‐one 3,30‐diacetate (trevoagenin A diacetate) (2), its 20S‐isomer (trevoagenin B diacetate) (3) and their related (20R)‐3β,30‐diacetoxy‐16‐oxo‐25,26,27‐trisnordammarane‐24,20‐lactone (4) and its 20S‐isomer (5) have been achieved. Suitable tetrahydrofuran models have been synthesized in order to aid the13C NMR assignments of the side‐chain carbons of the above‐mentioned compounds. The remarkable chemical shift differences observed for C‐21 and C‐22 between each pair of the C‐20 epimers (2, 3 and 4, 5) allowed the confirmation of the C‐20 stereochemistry

ISSN:0030-4921    

DOI:10.1002/mrc.1270220108

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractA computer‐assisted method of conformational analysis for porphyrin molecules bearing flexible side‐chains has been developed. The method utilizes the ring current‐induced chemical shift changes of the side‐chain protons which arise from the porphyrin macrocycle and any attached aryl rings. The treatment has been applied to a series of carotenoporphyrin molecules, which are important as models for a variety of photophysical processes in biological systems. Chemical shift data of sufficient accuracy for the conformational analysis were obtained from 500 MHz NMR experiments. The conformations of the carotenoporphyrins varied from extended ones with the carotenoid well away from the porphyrin ring to tightly folded species, depending on molecular constitution. The analytical method can be extended to other porphyrin‐base

ISSN:0030-4921    

DOI:10.1002/mrc.1270220109

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractAll resolvable13C13C Spin–Spin Coupling constants in 1‐ and 2‐methylnaphthalene have been measured in natural abundance by the two‐dimensional INADEQUATE method. The results have been interpreted in terms of HMO

ISSN:0030-4921    

DOI:10.1002/mrc.1270220110

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY

摘要:

AbstractThe ring mobility of some 1,5‐benzodiazepine‐2,4‐diones and 1,4‐benzodiazepin‐2‐one has been investigated by the temperature dependence of their1H NMR spectra. The activation parameters have been obtained, and the influence of N‐1 substitution on the ring mobility is discussed, taking into account previously reported data for simi

ISSN:0030-4921    

DOI:10.1002/mrc.1270220111

出版商:John Wiley&Sons Limited    

年代:1984

数据来源: WILEY